A sector is a specifically sized division of a disk drive. A block is a group of sectors that can be accessed (e.g., read, written, or both) by an operating system of a host computer. Common disk drives hold 512 bytes or 528 bytes of data in a sector. Certain new disk drives hold 4,096 bytes or 4,224 bytes of data in a sector, generally referred to as 4 kilobytes (4 KB). An operating system of a host computer can build a request with a block size based on the smaller sector size even when the disk drive has a 4 KB sector size. When this block size is not a multiple of the disk drive's sector size, then a storage controller will need to read the disk drive before writing that data.
Count-key-data (CKD) is a disk data organization model of certain operating systems. CKD architecture derives its name from the record format that typically includes a field containing the number of bytes of data and a record address, an optional key field, and the data, itself. CKD records are typically stored in a block size of 512 bytes or multiples of 512 bytes. Thus, operating systems often seek to access records in less than the conventional 4 KB block size.
RAID controllers running RAID-5 or RAID-6 read the old disk drive before writing the new data to the disk drive so it can generate P parity and Q parity. Thus, when the write operation block size is not a multiple of the disk drive sector size, the storage controller does not need to issue extra disk drive operations, but it may need an efficient data flow.